1. The Field of the Invention
The invention is in the field of concrete compositions, more particularly in the design-optimization of concrete compositions based on factors such as performance and cost. The invention more particularly relates to the design and manufacture of concrete using improved methods that more efficiently utilize all the components from a performance and cost standpoint and minimize strength variability, as well as unique methods for redesigning an existing concrete mix design and upgrading the batching, mixing and/or delivery system of an existing concrete manufacturing plant.
2. The Relevant Technology
Concrete is a ubiquitous building material. Finished concrete results from the hardening of an initial cementitious mixture that typically comprises hydraulic cement, aggregate, water, and optional admixtures. The terms “concrete”, “concrete composition” and “concrete mixture” shall mean either the finished, hardened product or the initial unhardened cementitious mixture depending on the context. It may also refer to the “mix design”, which is the formula or recipe used to manufacture a concrete composition. In a typical process for manufacturing transit mixed concrete, the concrete components are added to and mixed in the drum of a standard concrete delivery truck, typically while the truck is in transit to the delivery site. Hydraulic cement reacts with water to form a binder that hardens over time to hold the other components together.
Concrete can be designed to have varying strength, slump, and other materials characteristics, which gives it broad application for a wide variety of different uses. The raw materials used to manufacture hydraulic cement and concrete are relatively inexpensive and can be found virtually everywhere although the characteristics of the materials can vary significantly. This allows concrete to be manufactured throughout the world close to where it is needed. The same attributes that make concrete ubiquitous (i.e., low cost, ease of use, and wide availability of raw materials) have also kept it from being fully controlled and its full potential developed and exploited.
Concrete manufacturing plants typically offer and sell a number of different standard concrete compositions that vary in terms of their slump and strength. Each concrete composition is typically manufactured by following a standard mix design, or recipe, to yield a composition that has the desired slump and that will harden into concrete having the desired strength. Unfortunately, there is often high variability between the predicted (or design) strength of a given mix design and the actual strength between different batches, even in the absence of substantial variability in the quality or characteristics of the raw material inputs. Part of this problem results from a fundamental disconnect between the requirements, controls and limitations of “field” operations in the concrete batch plant and the expertise from research under laboratory conditions. Whereas experts may be able to design a concrete mixture having a predicted strength that closely reflects actual strength when mixed, cured and tested, experts do not typically prepare concrete compositions at concrete plants for delivery to customers. Concrete personnel who batch, mix and deliver concrete to job sites inherently lack the ability to control the typically large variation in raw material inputs that is available when conducting laboratory research. The superior knowledge of concrete by laboratory experts is therefore not readily applicable or transferable to the concrete industry in general.
In general, concrete mixtures are designed based on such factors as (1) type and quality of hydraulic cement, (2) type and quality of aggregates, (3) quality of water, and (4) climate (e.g., temperature, humidity, wind, and amount of sun, all of which can cause variability in slump, workability, and strength of concrete). To guarantee a specific minimum strength and slump as required by the customer (and avoid liability in the case of failure), concrete manufacturers typically follow a process referred to as “overdesign” of the concrete they sell. For example, if the 28 day field strength of a particular concrete mix design is known to vary between 2500 psi and 4000 psi when manufactured and delivered, a manufacturer must typically provide the customer with a concrete composition based on a mix design that achieves a strength of 4000 psi under controlled laboratory conditions to guarantee the customer a minimum strength of 2500 psi through the commercial process. Failure to deliver concrete having the minimum required strength can lead to structural problems, even failure, which, in turn, can leave a concrete plant legally responsible for such problems or failure. Thus, overdesigning is self insurance against delivering concrete that is too weak, with a cost to the manufacturer equal to the increased cost of overdesigned concrete. This cost must be absorbed by the owner, does not benefit the customer, and, in a competitive supply market, cannot easily be passed on to the customer.
Overdesigning typically involves adding excess hydraulic cement in an attempt to ensure a minimum acceptable strength of the final concrete product at the desired slump. Because hydraulic cement is typically the most expensive component of concrete (besides special admixtures used in relatively low amounts), the practice of overdesigning concrete can significantly increase cost. However, adding more cement does not guarantee better concrete, as the cement paste binder is often a lower compressive strength structural component compared to aggregates and the component subject to the greatest dynamic variability. Overcementing can result in short term microshrinkage and long term creep. Notwithstanding the cost and potentially deleterious effects, it is current practice for concrete manufacturers to simply overdesign by adding excess cement to each concrete composition it sells than to try and redesign each standard mix design. That is because there is currently no reliable or systematic way to optimize a manufacturer's pre-existing mix designs other than through time-consuming and expensive trial and error testing to make more efficient use of the hydraulic cement binder and/or account for variations in raw material inputs.
The cause of observed strength variability is not always well understood, nor can it be reliably controlled using existing equipment and following standard protocols at typical ready-mix manufacturing plants. Understanding the interrelationship and dynamic effects of the different components within concrete is typically outside the capability of concrete manufacturing plant employees and concrete truck drivers using existing equipment and procedures. Moreover, what experts in the field of concrete might know, or believe they know, about concrete manufacture, cannot readily be transferred into the minds and habits of those who actually work in the field (i.e., those who place concrete mixtures into concrete delivery trucks, those who deliver the concrete to a job site, and those who place and finish the concrete at job sites) because of the tremendous difference in controls and scope of materials variation. The disconnect between what occurs in a laboratory and what actually happens during concrete manufacture can produce flawed mix designs that, while apparently optimized when observed in the laboratory, may not be optimized in reality when the mix design is scaled up to mass produce concrete over time.
Besides variability resulting from poor initial mix designs, another reason why concrete plants deliberately overdesign concrete is the inability to maintain consistency of manufacture. There are four major systemic causes or practices that have historically lead to substantial concrete strength variability: (1) the use of materials that vary in quality and/or characteristics; (2) the use of inconsistent batching procedures; (3) overcementing; and (4) adding insufficient batch water initially and later making slump adjustments at the job site, typically by the concrete truck driver adding an uncontrolled amount of water to the mixing drum. The total variation in materials and practices can be measured by standard deviation statistics.
The first cause of variability between theoretical and actual concrete strengths for a given mix design is variability in the supply of raw materials. For example, the particle size, size distribution, morphology, and particle packing density of the hydraulic cement and aggregates (e.g., course, medium, and fine) may vary from batch to batch. Even slight differences can greatly affect how much water must be added to yield a composition having the required slump. Because concrete strength is highly dependent on the water-to-cement ratio, varying the water content to account for variations in the solid particle characteristics to maintain the required slump causes substantial variability in concrete strength. Unless a manufacturer can eliminate variations in raw material quality, overdesigning is generally the only available way to ensure that a concrete composition having the required slump also meets the minimum strength requirements.
Even if a concrete manufacturer accounts for variations in raw materials quality, overdesigning is still necessary using standard mix design tables. Standardized tables are based on actual mix designs using one type and morphology of aggregates that have been prepared and tested. They provide slump and strength values based on a wide variety of variables, such as concentration of cement, aggregates, water, and any admixtures, as well as the size of the aggregates. The use of standardized tables is fast and simple but can only approximate actual slump and strength even when variations in raw materials are measured. That is because the number of standardized mix designs is finite though the variability in the type, quality and concentration (i.e., ratio) of raw materials is virtually infinite. Because standardized tables can only approximate real world raw material inputs, there can be significant variability between predicted and actual strength when using mix designs from standardized tables. Because of this variability, the only two options are (1) time consuming and expensive trial and error testing to find an optimal mix design for every new batch of raw materials or (2) overdesigning. Manufacturers typically opt for overdesigning, especially in light of factors other than mix design that cause variations between design and actual strength.
The second cause of strength variability is the inability to accurately deliver the components required to properly prepare each batch of concrete. Whereas modern scales can theoretically provide very accurate readings, sometimes to within 0.05% of the true or actual weight, typical hoppers and other dispensing equipment used to dispense the components into the mixing vessel (e.g., the drum of a concrete mixer truck) are often unable to consistently open and shut at the precise time in order to ensure that the desired quantity of a given component is actually dispensed into the mixing vessel. To many concrete manufacturers, the perceived cost of upgrading or properly calibrating their metering and dispensing equipment is higher than simply overdesigning the concrete, particularly since most manufacturers have no idea how much the practice of overdesigning concrete actually costs and because it is thought to be a variable cost rather than a capital cost.
Overdesigning often leads to the third cause of strength variability, which is overcementing. Overcementing involves increasing the amount of hydraulic cement in an attempt to achieve or guarantee a minimum strength by overcoming the effect on strength by randomly adding water after batching to adjust slump. This, however, can lead to increases in strength variability, as hardened cement paste is typically weaker as a structural element compared to the aggregate components. While adding more cement may increase the binding strength provided by the cement paste that holds the aggregates together, more cement can also weaken concrete by displacing stronger aggregate materials with the weaker cement paste as a structural component of the hardened concrete. Strength variability occurs as a result of the foregoing effects working in opposite directions, but in differing amounts between different batches of concrete (e.g., due to differences in the water-to-cement ratio, quality and characteristics of the hydraulic cement, aggregates and water, and how the concrete is handled when delivered to a job site).
Overcementing can also cause microshrinkage, particularly on or near the surface due to water evaporation, which reduces the strength and durability of the concrete surface. Microshrinkage caused by overcementing and poor component distribution can cause cracks and crazing within 1-2 years of manufacture. Overcementing can also cause creep, which is the dynamic (and usually undesirable) growth of concrete masses due to continued long term hydration and growth of hydration products of the cement grains,
The fourth cause of concrete strength variability is the practice by concrete truck drivers of adding water to concrete after batching in an attempt to improve or modify the concrete to make it easier to pour, pump, work, and/or finish. In many cases, concrete is uniformly designed and manufactured to have a standard slump (e.g., 3 inch) when the concrete truck leaves the lot, with the expectation that the final slump requested by the customer will be achieved on site through the addition of water. This procedure is imprecise because concrete drivers rarely, if ever, use a standard slump cone to actually measure the slump but simply go on “look and feel”. Since adding water significantly decreases final concrete strength, the concrete plant must build in a corresponding amount of increased initial strength to offset the possible or expected decrease in strength resulting from subsequent water addition. Because strength can be decreased by varying amounts depending on the actual amount of water added by the driver, the manufacturer must assume a worst-case scenario of maximum strength loss when designing the concrete in order to ensure that the concrete meets or exceeds the required strength.
Given the foregoing variables, which can differ in degree and scope from day to day, a concrete manufacturer may believe it to be more practical to overdesign its concrete compositions rather than account and control for the variables that can affect concrete strength, slump and other properties. Overdesigning, however, is not only wasteful as an inefficient use of raw materials, sometimes providing concrete that is substantially stronger than what is required can also be dangerous. For example, because stronger concrete is often more brittle than weaker concrete, it can fail before the weaker concrete when subjected to the forces of an earthquake.
In an effort to more efficiently design concrete compositions and take into account variations in the particle size, particle size distribution, morphology, and packing densities of the various solid components between different batches of cement and aggregates, the inventors previously developed a design optimization process that greatly improved upon traditional methods for designing concrete mixtures. This process is described in U.S. Pat. No. 5,527,387 to Andersen et al., entitled “Design Optimized Compositions and Computer Implemented Processes for Microstructurally Engineering Cementitious Mixtures” (hereinafter “Andersen patent”). For brevity, the design optimization process disclosed in the Andersen patent will be referred to as the “DOC program” (the term “DOC” being an acronym for “design optimized concrete”).
The DOC program mathematically relates the properties of strength, slump and other aspects, such as cost, cohesiveness and durability, based on the concentrations and qualities of the various raw material inputs. The DOC program is able to design and virtually “test” millions of different hypothetical mix designs in seconds using a computer. This greatly reduces the amount of time required to carry out trial-and-error testing that would otherwise be necessary to identify a concrete mixture that is optimized for strength, slump, cost and/or other desired features. The goal of the DOC program is to identify an optimal mix design, from among a large number of hypothetical mix designs, based on such desired features as slump, strength, and cost. The DOC program fills in gaps inherent in standardized tables, which include a relatively small number of mix designs given the variability of raw material inputs. The DOC program can design and virtually “test” millions of different mix designs, including those falling between the gaps of standardized tables, in much less time than it takes to design and test one mix design using conventional trial-and-error methods.
First, the raw materials are carefully tested to determine characteristics that affect the slump, strength, cost, and/or other desired features of cementitious compositions made therefrom. These include, for example, the particle size and packing density of the various aggregate components (e.g., large, medium and small aggregates) and hydraulic cement particles, and the effect of one or more optional admixtures (e.g., fly ash, water reducers, fillers, etc.). Once the raw materials have been characterized with the required degree of accuracy, their characteristics are input into a computer used to carry out the optimization process of the DOC program.
Thereafter, the DOC program designs a large number of hypothetical concrete mixtures, each having a theoretical slump and strength, by varying the concentrations of cement, aggregate, water, and optional admixtures. The predicted slump and strength of each hypothetical concrete mixture is determined by inputting the variables (e.g., the concentration and characteristics of the raw materials) into a system of interrelated mathematical equations. One of the equations utilized in the DOC program is a variation of Feret's strength equation, which states that the compressive strength of the final hardened concrete composition is proportional to the square of the volumetric ratio of hydraulic cement to cement paste, which consists of cement, water and air:
  σ  =      K    ·                  (                              V            C                                              V              C                        +                          V              W                        +                          V              A                                      )            2      
The constant “K” within this equation provides proper strength units and magnitude. The strength equation can be modified as follows to predict the strength of concrete that additionally includes other binders, such as class F fly ash, as part of the cement paste:
  σ  =      K    ·                  (                                            V              C                        +                          0.3              ⁢                              V                FA                                                                        V              C                        +                          0.3              ⁢                              V                FA                                      +                          V              W                        +                          V              A                                      )            2      
The DOC program can be carried out in an iterative manner in which each iteration yields a hypothetical concrete mixture having a predicted slump and strength that is closer to the desired slump and strength than each previous iteration. In addition to slump and strength, the DOC program can optimize concrete for other desired features, such as cost, workability, or cohesion. Thus, in the case where a number of different concrete mixtures may have the desired slump and strength, the DOC program can identify which of the mixtures is “optimal” according to one or more other criteria (e.g., cost, workability and/or cohesion).
Notwithstanding the foregoing, the DOC program, when initially invented, was based on the assumption, well-accepted in the art, that the constant K (or “K factor”) within Feret's strength equation is a true constant and does not vary as long as the same type of mixing apparatus and source of raw materials are used each time. It has been well-accepted in the art that if such variables are kept constant, the K factor remains constant regardless of variations in hydraulic cement concentration and concrete strength. As a result of this well-accepted assumption, the DOC program required significant post-design corrections, even significant testing and redesign of concrete compositions made using one or more of the “optimal” mix designs generated by the program. Thus, the inability of the DOC program to account for dynamic variability of the K factor limited the practical application of an otherwise powerful design optimization tool.